1. Field of the Invention
The present invention relates to an imaging optical system having variable focusing function for an imaging lens, more particularly to an imaging optical system for use in an endoscope which is functioning as a variable focusing element and is useful in case of constructing an imaging element required for a compact imaging optical system, such as an endoscope or the like.
2. Related Art Statement
As a conventional imaging optical system for use in an endoscope for an imaging lens or the like having automatic infocusing switching function, there is an imaging optical system described in, for example, Japanese Patent Application Publication No. 35,090/87. This publication discloses that a holding frame of an objective lens mounted within a range of a distal hard portion of the endoscope is so formed as to extend the frame in the longitudinal direction, thereby moving the frame back and forth mechanically in the longitudinal direction by the operation for focusing adjusting at proximal operating section, resulting in a change of a pint position.
In the conventional imaging optical system disclosed in Japanese Patent Application Opened No. 46,423/90, the imaging optical system comprises a mechanism for selecting polarizing directions and a variable focusing lens utilizing an electro-optical effect of a liquid crystal, thereby changing the focal length of the objective lens by an electrical drive.
The above conventional imaging optical system disclosed in Japanese Patent Application Publication No. 35,090/87, discloses that the mechanism for moving the holding frame mechanically is accommodated in a minimum space, such as a distal hard section for the endoscope, but it is substantially impossible to obtain such a construction due to the spatial restriction.
The above conventional imaging optical system disclosed in Japanese Patent Application Publication No. 46,423/90, has a requirement of a function for selecting the polarizing directions in order to prevent double images from being generated due to the birefringent of the liquid crystal, so that transmittance of the vari-focus lens becomes decreased to 50% or less. Such a decrease of light quantity becomes fatal to the lens for an endoscope, so that usually, as a countermeasure, the diameter of an aperture diaphragm is increased or the diameter of light guide is increased. The former, however, has a defect that the focal depth becomes shallow, and the later has a defect in that as the diameter of the endoscope itself increases. Therefore, both countermeasures are not practical to accommodate such variable focusing function in the endoscope.